Manufacture of rayon



June 6, 1961 c. L. HENRY MANUFACTURE OF RAYON Filed Dec. 26, 1957 STRESS- STRAIN DIAGRAM w 8 T .l 4 h n W; a O M 0. Y 5 .nr 2 N yn 06 a aw m my mmC .fib v do w O mn nma v O a a m nl m 2 h d-T D. ms c w ATMAMAS 1 2 3 4 O 5 2 O 1/ P P 5 O. O 3 2 2 l l o O ELONGATION INVENTOR. CHARLES L. HENRY BY ya m, rmzzh ATTORNEY United States Patent 2,987,371 MANUFACTURE OF RAYON Charles L. Henry, Candler, N.C., assignor to American Enka Corporation, Enka, N.C., a corporation of Delaware Filed Dec. 26, 1957, Ser. No. 705,368 7 Claims. (Cl. 18-54) This invention relates to the manufacture of regenerated cellulose fibers, filamentary yarns and the like from viscose and particularly to the manufacture of viscose rayon especially characterized by low elongation, high tenacity and high modulus properties.

Although viscose rayon fabrics have superior characteristics to other types of fabrics, such as luster, softness and design, they lack adequate dimensional stability unless subjected to very expensive finishing treatments. Hence, it has been the goal of the rayon manufacturer to produce in an economical manner viscose rayon yarns and fibers that can be converted into fabrics with dimensional stability equaling or exceeding that of cotton fabrics. Undoubtedly, the dimensional stability of cotton fabrics is due to the combined effect of several basic fiber properties, the most important of these properties being strength, elongation and modulus. Since all of these properties come into consideration in a determination of the wet and dry stress-strain diagrams of a fiber, it is believed that if rayon yarns and fibers could be manufactured with a diagram which approximates that of a cotton fiber, rayon fabrics could be manufactured that are potentially more dimensionally stable.

Viscose rayon filamentary yarns and fibers now produced by conventional processes show elongation at the breaking point in the conditioned state higher than cotton fibers by about 50-100% and more. In the wet state, the elongation at the breaking point of these rayon yarns and fibers exceed that of cotton by about l-200% and more. As a result, conventional rayons, both regular tenacity and high tenacity types, show conditioned and wet tension moduli markedly lower than the corresponding moduli of cotton fibers. While the higher elongation of normal viscose rayon yarns and fibers is a desirable property in certain end uses, it is a definite disadvantage in many other uses. For example, a blend of cotton and conventional rayon is economically attractive and results in a more expensive appearing fabric; however, such blends cannot be made into an entirely satisfactory fabric since this fabric has considerably lower strength values than similar 100% cotton fabrics. This is due to the fact that in the aforementioned blend, the rayon fibers do not assume their share of the applied load before the cotton fibers have been extended to their breaking limit.

Methods of manufacturing regenerated cellulose yarns and fibers with elongations and moduli comparable to cotton are known, but in such cases they are either too brittle to be commercially useful or the manufacturing processes have serious economic disadvantages.

An object of the present invention is to provide a process for the manufacture of flexible, high tenacity viscose rayon filamentary yarns and fibers which have substantially lower elongations at the breaking point and markedly higher wet and dry tension moduli than normal rayon filaments with the process being substantially as economically attractive as conventional viscose manufacturing processes.

Another object of the present invention is to provide 2,987,371 Patented June 6, 1961 low elongation, high tenacity viscose rayon filamentary yarns and fibers having a high modulus while retaining their flexible or resilient character, and which, when used in blends with cotton and other fibers or used alone, canrbe made into fabrics having improved, novel,

and desirable physico-chemical properties and characteristics.

Other objects and advantages of the present invention will become apparent from the following detailed description.

It has been discovered that the aforesaid objects are accomplished by extruding viscose into an aqueous sulfuric acid-sodium sulfate-zinc sulfate primary spinbath under the following highly critical combination of spinning variables. The total alkali in the viscose is be tween 4.8-7.5% by weight, the concentration of Zinc sulfate in the spinbath is between 0.l-2.0% by weight, and the acid concentration (for any given zinc sulfate and viscose alkali combination within the prescribed limits) is within the range defined by the following equations:

After the filaments so produced are Withdrawn from the primary bath, they are highly stretched, preferably in 'a hot acidic secondary bath, and after-treated by the usual processes. Although selection of a combination of a low viscose alkali and a high zinc sulfate concentration within their specified limits would allow the use of an acid concentration as low as about 1.5-2.0% according to Equations 1 and 2, however, for best results, it is preferred to select a viscose alkali and zinc sulfate relationship which requires an acid concentration of about 3% or higher. Other spinning variables are correlative with the above described zinc sulfate, viscose alkali, and sulfuric acid relationship.

In the preparation of viscose according to this invention, the source of cellulose may be wood pulp, cotton linters, a mixture of the two or even some other source; high alpha wood pulps are preferred. The cellulose content of the viscose may range from about 50-85% with the preferred range being 6.08.0%. In the xanthation step, the amount of CS may be from 25-50% (based on oven-dry recoverable cellulose); however, the conventional quantities of 32-40% are quite satisfactory. The viscose is ripened to approximately the same level of maturity before spinning as in case of normal viscose used in the industry. For example, under suitably adjusted conditions, the Hottenroth maturity index of 7.3- 6.8 (percent cellulose-percent total alkali) viscose may vary at spinning from about 8-13; however, the preferred range is 9-11. Similarly, the more satisfactory range for a 7.4-5.6 viscose is about 7-9.

The primary spinbaths contain sulfuric acid, sodium sulfate and zinc sulfate; for special purposes, the use of magnesium sulfate or ferrous sulfate may be desirable. As shown by Equations 1 and 2, suitable acid concentrations are determined by the choice of viscose alkali and spinbath zinc sulfate concentrations. An outstanding characteristic of this process is that its acid concentrations generally are below the acid concentrations which will produce filaments with marked crimp developing properties. Upon increasing the acid in the spinbath to about the maximum allowed by Equation 2 and then above, the filaments will progressively acquire a nonuniform, cross-sectional skin area, and Will, as a result, crimp upon immersion in a relaxed state in hot water. Further increases in acid concentration will finally cause the crimping phenomenon to disappear; at this point the acid concentration is in the lower part of the acid range conventionally used for manufacture of viscose-yarns.

In the primary spinbath, zinc sulfate must be present to the extent of at least 0.1% unless replaced inwhole or in part by other divalent salts, such as ferrous sulfate or magnesium sulfate.

tions would be required in accordance with Equations 1 and 2. The concentration of Na SO may range from about 12% to about 26%, although 14-25% is preferable. The temperature of the primary bath should be between 30-70 C. with better operating efliciency and yarn properties resulting between 40-60 C.

After the viscose is extruded into the primary spinbath, the freshly formed filaments are led through the bath liquid for a distance of about 6 to about 200 inches; generally 8-20 inches are suflicient for fine denier yarns while 20-100 inches are usually adequate for heavy denier yarns and tows. The filaments are led from the bath to a feed wheel and thereafter stretched from about 50-100% and more. Stretching is preferably done in a secondary bath at a temperature of about 80-100 C. The secondary bath must contain sulfuric acid, preferably 13%. Preferably, the secondary bath may be made by diluting one part of the primary spinbath with two parts or more of water with sufiicient acid being added, if necessary, to bring the acid concentration within the aforesaid prescribed limits. After stretching, the filaments are collected at about 60-120 or more meters per minute in a rotating pot, or on a spool or bobbin, and aftertreated in a conventional manner. Optionally, the filaments may be aftertreated in a continuous manner and then collected.

In commercial practice when using the lower sulfuric acid concentrations and higher zinc sulfate concentrations permitted by Equations 1 and 2, the formation of ag glomerates and hard deposits of ZnS (contaminated with sulfur) and the like in the primary spinbath and on the walls of its container may occur. To prevent this, small quantities of suitable cation-active agents may be incorporated in the viscose and/or primary spinbath.

The yarns and fibers produced by the present process have a collection of unique physical properties heretofore not associated with rayon and which are comparable to cotton in most respects. The filaments are a high Quantities of ZnSO greater than 2.0% are undesirable as unfeasibly low acid concentratenacity type having a conditioned tenacity of about 3 g./ .1

denier and higher, and a tension modulus which approximates or exceeds that of cotton. Furthermore, this product is flexible and resilient and, accordingly, may be satisfactorily processed and converted into fabrics having improved serviceability. Staple fiber so produced is satisfactorily compatible with cotton for the manufacture of spun yarn. Fabrics produced from this spun yarn have strength and dimensional stability comparable to an allcotton fabric, and in addition, have a significantly improved appearance and hand. Furthermore, fabrics derived from 100% staple fibers only require the same resin treatment as do 100% cotton fabrics to impart substantially the same degree of dimensional stability and crease recovery.

The term tension modulus as used herein refers to the force required in grams per denier to elongate a single filament, fiber-or yarn by 5% Thus, the higher the value for the tension modulus the more resistant is the filament 0r fiber to being stretched or distorted.

The term flexible and resilient is used to describe a. filament that resists breakage or fracture when subjected to abrasive and/ or distortive forces; and its meaning is synonomous with the meaning of non-brittle.

To facilitate a better understanding of the present invention, reference is made to the annexed drawing wherein graphical illustrations of representative determinations of stress-strain diagrams for filamentary yarn and staple fiber produced in accordance with the present invention are compared with a typical cotton fiber and normally produced viscose rayon. FIGURE 1 represents moduli measurements made in a Wet state on these filaments with points p defining their respective tension modulus. As can be noted, the curves for the filaments show a tension moduli comparable to or higher than that for cotton and markedly higher than the modulus for normally produced yarn.

The invention will be more clearly understood by reference to the examples and discussions which follow. These examples are given for illustrative purposes only and are not to be construed as limitative. All concentrations used herein' unless otherwise designated are calculated as percentages by weight based on the viscose solution or the spinbath.

EXAMPLE I Alkali cellulose was prepared in the conventional manner from wood pulp and aged to obtain a degree of polymerization that would yield a viscose solution viscosity of 40-60 seconds by the ball fall method. The aged alkali cellulose was xanthated at about 27 C. for two hours using 36% CS (based on oven-dry recoverable cellulose). The resulting xanthate crumbs were dissolved in a solution of sodium hydroxide of a predetermined alkali concentration to produce a 7.36.82.25 (percent cellulose total alkali expressed as percent 'NaOH-percent 'total sulfur) viscose solution. After dissolving, the freshly prepared viscose solution was deaerated, filtered, and ripened to a Hottenroth maturity index of about 10.5.

The viscose solution was spun into four filamentary yarns; that is, yarns of (a) 100 denier-lOOfilaments, (b) 100 denier-60 filaments, (c) 150 denier-40 filaments and (d) 200 denier-40 filaments. The equipment and operating procedure for the spinning of these yarns were essenaqueous primary spinbath containing 5.0% H 18% Na SO and 1.0% ZnSO (maintained at a temperature of 50 C.); the first yarn listed above was manufactured by extruding the viscose through a spinneret with orifices .of 50 microns diameter, the second yarn was prepared using a spinneret with orifices of 60 microns diameter and the third and fourth yarns Were prepared using a spinneret with orifices of 75 microns diameter. After a travel of nine inches in the primary spinbath, the yarn was then withdrawn therefrom and led to the first feed wheel, thence through a secondary bath maintained at a temperature of C., then to a second feed wheel, and finally collected in a rapidly rotating pot. The secondary bath was diluted primary spinbath. The peripheral speeds of the first and second feed wheels were 38 and 80 meters per minute, respectively. Due to the difference in the peripheral speeds of the wheels, the yarn was stretched The collected yarn was washed free of acid and salts. After a finish was applied, the yarn was dried. The yarn was tested for breaking tenacity, breaking elongation and tension modulus (at 5% elongation) in both conditioned and wet states. The conditioned state was obtained by storing the yarn in a room at 75 F. and 60% relative humidity for 24 hours.

The properties of the above four yarns are listed in Table I. For comparative purposes, properties are also listed for two conventional viscose rayon textile yarns, and a super strength type rayon yarn which was commercially produced especially for =tire cords. The tension modulus (at 5% elongation) of the super type yarn was determined on single filaments while the modulus of all the other yarns was measured by testing all the filaments together.

Table I Table II Tenacity, Elongation, lVIodulus at A Conveng./d. Percent 5% Elonga- Sam- Sam- Mlddllllg tional tion, g./d. Single Fiber Properties ple ple Cotton Rayon 5 A B Staple Fiber Cond. Wet Cond. Wet Cond. Wet

Denier/fiber 1. 5 1. 5 2. 1. Yarn produced according Tenacity, Ereaklng:

to Example I: Conditioned, g [d 2. 9 3. 2 2.9 2. 5 100 denier-100 file- 1 Wet, g./d- 1.8 2.0 3.1 1.5

ments 3. 8 2. 3 8 8 2. 9 1. 3 Elongation, Breakin 100 denier-60 fila- Conditioned, Percent 11 11 11 23 men s i. 3. 6 2. 2 8 9 2.7 1. 1 Wet, Percent 14 12 -14 28 150 denier-40 fila- Modulus, 5% Elongation Wet,

ments 3.5 2.2 8 9 2.7 1.1 g./d 0. 55 0.65 0.40-0.80 0.20 200 denier40 filamerits 3. 4 2. 2 9 10 2. 6 1. 2 ooflvemixgfl'll Viscose 15 Table II shows that rayon staple fiber manufactured in igg lgfq accordance with this example has improved properties, e bright L0 19 27 L0 especially wet modulus, as compared with conventional 150 denier-40 fila- 1 ments, bright 2.0 1.0 21 27 0. 90 0.19 rayon Stall 1,900 dtenier:s720 m To study the compatrbrlrty of the stable fiber with cot- F1 I e igmia1d 25 33 20 ton as compared Wlth the compatibility of conventional rayon stable fiber with cotton, three yarns were spun on The values shown in Table I for the conditioned and wet moduli are remarkably high when compared to those of conventional rayon yarns. The elongation and modulus of these yarns are primarily determined by the spinning stretch, that is, a higher stretch induces lower elongation and higher modulus. Thus, these yarns can be conveniently produced with these two properties being easily adjusted for particular end uses.

EXAMPLE II A 7 .4-5.6-2.20 (percent cellulose-percent total alkalipercent total sulfur) viscose was prepared in a manner similar to that described in Example I. At a Hottenroth maturity index of about 8.0, the viscose was spun into 1100 denier, 720-filament yarn by extruding it into a 3.5% H SO 21% Na SO 1.0% ZnSO primary spinbath maintained at 40 C. Using an operating procedure similar to the one used in Example I, a stretch was imparted to the freshly spun yarn sufficient to cause a tension of 650 grams 0n the yarn just before the second feed wheel. The yarn after washing, finishing and drying had conditioned and wet breaking tenacities of 3.5 g./ denier and 2.1 g./ denier, respectively, and corresponding elongations of 9% and 10%. Conditioned tension modulus (at 5% elongation) was 2.3 g./ denier and the wet modulus was 0.80 g./ denier; the modulus determinations were made on single filaments extracted from the yarn.

EXAMPLE HI Two 7.45.6-2.20 viscoses were prepared in a manner similar to that described in Example I. After aging to a Hottenroth maturity index of about 7.5, each viscose was extruded into 11,000 denier, 7,500 filament tows in a primary spinbath maintained at 50 C. and composed of 4.5% H SO 21% Na SO and 1.0% ZnSO After 45 inches travel in the primary bath, the tows were Withdrawn therefrom by means of a first feed Wheel. Then the tows were guided into and through a second weakly acidic bath at a temperature of 90 C. and Withdrawn therefrom by means of a second feed wheel. Due to a predetermined differential in the peripheral speeds of said feed wheels, the first tow was stretched 89% with the second tow being stretched 98%.

The first tow was cut into staple fiber of 1.25 inch lengths and is hereafter designated Sample A. The second tow was also cut into staple fiber of 1.25 inch lengths and is hereafter designated Sample B.

The fibers of Samples A and B were washed free of acid and salts. After a finish was applied, the fibers were dried. Physical properties of these fibers are shown in Table 11 along with properties of Middling cotton and of a commercially produced, conventional rayon staple fiber,

the cotton system as indicated below:

Spun Yarn A: 30/1 yarn was spun from carded Middling cotton.

Spun Yarn B: 30/1 yarn was spun from a blend of /a staple fiber (Sample A) and /3 carded Middling cotton.

Spun Yarn C: 30/1 yarn was spun from a blend of /3 conventional rayon staple fiber and /3 carded Middling cotton.

The process of spinning Spun Yarn B processed in an entirely satisfactory manner. Fiber breakage during carding, roving, etc., was normal which showed that the fibers are not brittle. The properties of the three spun yarns are compared in Table III.

Table III Spun Spun Spun Spun Yarn Properties Yarn Yarn Yarn A B O Tenacity, Breaking:

Conditioned, g./d 1. 60 1.50 1. 25 We g. 2.10 1. 60 1. 40 Elongations, Breaking:

Conditioned, percent 6. 2 6.0 5. 5 Wet, percent 10. 4 10.3 9. 6 Skein Break Factor" Conditioned- 2, 065 2, 005 1, 620 Wet i. 2, 735 2, 255 1, 955

As shown by this data, the staple fiber is markedly more compatible with cotton than is conventional rayon staple fiber; the higher modulus of the former is probably the principal causative factor of the improved compatibility.

For comparative studies of properties including inherent dimensional stability (shrinkage) and susceptibility to dimensional stabilization by resin treatment, three fabrics were woven as below.

Fabric A: Woven from spun yarn of 100% carded Middling cotton.

Fabric B: Woven from spun yarn of 100% staple fiber of this example.

Fabric C: Woven from spun yarn of 100% conventional rayon staple fiber.

Specifications of these fibers (Greige state) are listed in 7 After a plain finishing treatment consisting of singeing, boiling OE, and drying relaxed, these fabrics were washed repeatedly and their dimensional stability measured, the results of which are shown in Table V.

Thus, according to filling and area shrinkage data, fabric derived from staple fiber of this example, compares favorably with the shrinkage of an all-cotton fabric, and is 25 vastly superior in all shrinkage characteristics to fabric derived from conventional rayon staple fiber.

Cuts ocf Fabric A and Fabric B were padded through a 15% dimethylol ethylene urea resin bath, dried on a tenter frame, cured, scoured, dried relaxed, and steam 3 framed. After the properties of these two fabrics Were measured, the fabrics were washed repeatedly and their dimensional stability measured. Results of these meas urements are shown in Table VI.

35 Table VI Properties and Dimensional Stability Fabric Fabric of Resin Treated Fabrics A B Unwashed Fabrics: 40

1. Ravel Strip Test- Warp strength, dry, lbs 36 57 Filling strength, dry, lbs 43 Warp strength, wet, lbs 39 Filling strength. wet, lbs 22 28 Warp elongation, dry, percent 8.9 13. 1 Filling elongation, dry, percen 18.0 24. 9 45 Warp elongation, wet, percent 101 14. 3 Filling elongation, wet, percent..- 20. 7 23. 8 2. Trapezoidal Tear Test Warp strength. dry, lbs 1. 9 5. 7 Filling strength, dry, lbs 1. 6 4. 7 8. Monsanto Crease Recovery- Warp, percent 63 64 Filling, percent 72 62 Washed Fabrics:

1. Dimensional Retention After One Wash In commercial practice, a 15% resin treatment of the resin-forming material used in the present example when applied to cotton fabrics is usually sufiicient to impart adequate dimensional stability and crease recovery, whereas even a 30% resin treatment applied to a conventional 5 rayon fabric does not impart these properties to the desired extent. However, the data in Table VI show that a 15 resin treatment applied to a fabric manufactured from the staple fiber is sufficient to produce dimensional stability and crease recovery substantially equivalent to that of a 15 resin treated cotton fabric. Further, as the application of the resin treatment to the cotton fabric caused a greater loss in fabric strength than did the application of the same resin to the fabric derived from staple fiber of this example, the ravel strip strength and EXAMPLE IV A viscose containing 7.8% cellulose, 5.0% total alkali and 2.30% total sulfur was prepared in a manner similar to that described in Example I. At a Hottenroth maturity index of about 8.0, the viscose was extruded as 1100 denier, 720-filament yarn into a 4.0% H 50 23% Na SO and 1.0% ZnSO primary spinbath maintained at 40 C. Stretch was applied in a secondary bath sufficient to cause a tension of about 650 grams on the yarn just before the second feed wheel. The yarn collection speed was 75 meters per minute with the yarn being collected in a rapidly rotating pot. After normal aftertreatment, the yarn had a conditioned tenacity of 3.4 g./ denier, a conditioned elongation of 13%, a wet tenacity of 1.9 g./denier and a wet elongation of 16.5%. Conditioned and wet moduli (at 5% elongation) were 1.8 g./ denier and 0.45 g./denier, respectively.

EXAMPLE V Table VII Yarn properties First Second Third Bath Bath Bath Tenacity, Breaking:

Conditioned, g./d 3. 6 3. 7 3. 6

Wet, g. d 2.4 2.3 2. 3 Elongation, Breaking:

Condition, percent 8.2 8.2 8. 2

Wet, percent 9. 2 9. 2 9.3 Modulus (5% Elongation):

Conditioned, g./d 2. 7 2. 8 2. 8

Wet, g./d 1. 7 1. 4 1.1

EXAMPLE VI A viscose of the same composition and maturity as used in Example I was extruded into 100 denier, 60-filament yarn in three primary spinbaths having different Na SO concentrations. The first primary spinbath contained 5.5% H 14% Na SO and 1.0% ZnSO the second contained 5.5% H SO 18% Na SO and 1.0% ZnSO and the third contained 5.5% H SO 22% Na SO and 1.0% ZnSO The other operating conditions were the same as described in Example 1. Properties of these yarns are listed in Table VIII.

Table VIII Yarn Properties First Second Third Bath Bath Bath Tenacity, Breaking:

lvonditigned, g./d

et, g. Elongation, Breaking:

Conditioned, Percent Wet, Percent Modulus (5% Elongation Conditioned, g./d Wet, g./d

EXAMPLE VII A viscose containing 7.3% cellulose, 6.8% total alkali, and 2.25% total sulfur was ripened to a Hottenroth maturity index of about 10.5 and spun into denier, 60- filament yarn in a manner similar to that described in Example I except that the primary spinbath was composed of 4.5% H SO 18% Na SO and 1% ZnSO The yarn collection speed was 115 meters per minute. The conditioned yarn had a tenacity of 3.7 g./denier and an elongation of 7.8%; the wet yarn had a tenacity of 2.2 g./ denier and an elongation of 7.6%. Yarn modulus (5% elongation) was 2.9 g./ denier for the conditioned state and 1.3 g./denier for the wet state.

EXAMPLE VIII At a Hottenroth maturity index of about 10.5, a viscose containing 7.3% cellulose, 6.8% total alkali 'and 2.25% total sulfur was spun into 100 denier, 60-filament yarn in a spinbath composed of 4.5% H 80 18% Na SO and 1.0% ZnSO and maintained at 60 C. The yarn was stretched 95% in a secondary bath maintained at 90 C. Other operating conditions were similar to those of Example I. The conditioned and wet tenacities of the yarn were 3.4 g./denier and 2.0 g./ denier, respectively; the corresponding elongations were 8.9% and 9.4%. The yarn showed a modulus (5% elongation) of 2.4 g./ denier in the conditioned state and 1.0 -g./ denier in the wet state.

EXAMPLE IX Alkali cellulose was aged so as to give a viscose solution viscosity of about 40 seconds by the ball fall method. The alkali cellulose was xanthated at 25 C. for two hours with 36% CS (based on oven-dry recoverable cellulose). The resulting cellulose xanthate crumbs were dissolved at 20 C. in a sodium hydroxide solution to produce a viscose having a composition of 6.0% cellulose, 6.0% total alkali, and 1.8% total sulfur. After deaerating and filtering, the viscose was ripened to a Hottenroth maturity index of about 11.0 and then spun into 100 denier, 60-filament yarn. The primary spinbath was composed of 4.0% H SO 18% Na SO and 1.0% ZnS the other operating conditions were similar to those of Example I. The finished yarn had a conditioned tenacity of 3.9 g./ denier, a wet tenacity of 2.5 g./denier, a conditioned elongation of 8.0% and a wet elongation of 8.2%. Conditioned modulus elongation) of the yarn was 2.8 g./denier and the wet modulus was 1.3 g./ denier.

While preferred embodiments of the invention have been shown, it is to be understood that changes and variations may be made herein without departing from the spirit and scope of the invention as defined by the following claims.

What is claimed is:

1. A process for producing high strength, low elongation, viscose rayon yarn and fiber which comprises extruding a viscose solution having an alkali content between 4.8-7.5 by weight, into filaments in an aqueous acid bath containing Na SO 0.1-2.0% by weight of ZnSO and H 50 in a concentration which, for any given viscose alkali and ZnSO, combination within their respective ranges, is expressed by the following equations:

(1) Minimum percent H SO =(percent viscose alkali 1.0) 2.0 {/percent ZnSO.,

(2) Maximum percent H SO =(percent viscose alkali +0.5) percent ZnSO, Vpercent ZnSO.,.

2. A process as defined in claim 1 wherein the viscose alkali and the ZnSO, concentrations require the use of an H SO concentration of at least 3% by weight.

3. A process for producing high strength, low elongation, viscose rayon yarn and fiber which comprises extruding viscose having an alkali content between 4.8- 7.5% by weight and a cellulose content of about 5.0- 8.5% by weight, into filaments in an aqueous acid bath containing Na SO 0.1-2.0% by weight of ZnSO and H 80, in a concentration which, for any given viscose alkali and ZnSO, combination within their respective ranges, is expressed by the following equations:

(1) Minimum percent H SO =(percent viscose alkali l.0) 2.0 {/percent ZnSO.,

(2) Maximum percent H SO =(percent viscose alkali +0.5) percent ZnSO Vpercent 21180 4. A process for producing high strength, low elongation, viscose rayon yarn and fiber which comprises extruding viscose having an alkali content between 4.8- by weight and a cellulose content of about 6.0- by Weight, into filaments in an aqueous acid bath at a temperature of about 30-70 C. containing about 1226% by weight of N21 SO 0.l2.0% by weight of ZnSO and H 50, in a concentration which, for any given viscose alkali and ZnSO, combination within their respective ranges, is expressed by the following equations:

(1) Minimum percent H SO =(percent viscose alkali 1.0) 2.0 V percent ZnSO,,

(2) Maximum percent H SO =(percent viscose alkali +0.5) percent ZnSO, x/percent ZnSO,.

(1) Minimum percent H SO =(percent viscose alkali -l.0) 2.0 Vpercent ZnSO, (2) Maximum percent H SO =(percent viscose alkali +0.5) percent ZnSO /percent ZnSO.;.

6. A process as defined in claim 5 wherein the filaments are stretched at least 50%.

7. A process as defined in claim 5 wherein the filaments are stretched at least 50% in a second hot weakly acidic bath.

References Cited in the file of this patent UNITED STATES PATENTS 2,515,834 Nicoll July 18, 1950 2,581,835 Cox Jan. 8, 1952 2,594,496 Richter Apr. 29, 1952 2,607,955 Drisch Aug. 26, 1952 2,612,679 Ladisch Oct. 7, 1952 2,674,025 Ladisch Apr. 6, 1954 2,732,279 Tachikawa Jan. 24, 1956 2,775,505 Pedlow Dec. 25, 1956 2,781,275 Baarn Feb. 12, 1957 OTHER REFERENCES Harris Handbook of Textile Fibers, Harris Research Laboratories, Inc., Washington, DC. (1954) pages 130, 131, 133. 

1. A PROCESS FOR PRODUCING HIGH STRENGHT, LOW ELONGATION, VISCOSE RAYON YARN AND FIBER WHICH COMPRISES EXTRUDING A VISCOSE SOLUTION HAVING AN ALKALI CONTENT BE TWEEN 4.8-7.5% BY WEIGHT, INTO FILAMENTS IN AN AQUEOUS ACID BATH CONTAINING NA2SO4, 0.1-2.0% BY WEIGHT OF ZNSO4, AND H2SO4 IN A CONCENTRATION WHICH, FOR ANY GIVEN VISCOSE ALKALI AND ZNSO4 COMBINATION WITHIN THEIR RESPECTIVE RANGES, IS EXPRESSED BY THE FOLLOWING EQUATIONS: 